Heretofore electromechanical sirens have been employed for producing very high sound volumes. Such a siren comprises a so-called chopper which is rotatably driven by an electric motor and which produces air pressure pulses at a sonic frequency. An electromechanical siren can be readily designed for a power input on the order of 15 to 25 Kw. With such power, and being reasonably efficient in converting electrical energy into sound energy, an electromechanical siren can produce a tone signal that is readily audible at distances of more than a mile.
An important disadvantage of an electromechanical warning apparatus is that it can produce only tone outputs and can therefore signal only a very limited number of conditions and/or instructions. Because of this limitation, there has been an increasing use of electronic apparatus for outdoor public alerting and warning, capable of producing both siren-type tone outputs and highly amplified voice outputs. In the case of an alerting and warning apparatus installed, for example, in connection with a chemical plant or a nuclear electric power plant, it is not sufficient merely to warn the public that a potentially hazardous condition has developed; the public must also be informed about the protective measures that should be taken under the circumstances, because the measures that are appropriate will differ from case to case depending upon the nature and severity of the hazardous condition and upon ambient conditions such as wind direction. For such situations the needed information can be satisfactorily conveyed only in highly amplified voice transmissions.
An outdoor electronic alerting and warning apparatus comprises one or more loudspeakers or drivers, each having a diaphragm that is usually enclosed in a rigid housing which cooperates with the diaphragm to define in front of it an acoustical impedance chamber with a restricted output port. The housing also defines a closed chamber behind the diaphragm that contains a permanent magnet secured to the housing and a coil secured to the diaphragm. When the coil is energized with an alternating current, it cooperates with the magnet to cause vibration of the diaphragm that imparts acoustical energy to the air in front of the diaphragm.
If the diaphragm of such a driver were to confront free air, very little load would be imposed upon it, and the excursions of the diaphragm, controlled only by its own stiffness and mass, could attain such amplitude that the diaphragm would be damaged. Instead, the acoustical impedance chamber in front of the diaphragm, with its restricted output port, imposes a load upon the diaphragm that limits its excursions to safe amplitude values. The impedance chamber also improves transfer of energy from the diaphragm to the free air. In heretofore conventional apparatus, such energy transfer is further improved by connecting the driver to an appropriately designed horn that has a narrow throat portion and diverges to a flaring mouth. The outlet port of the driver impedance chamber opens coaxially into the throat portion of the horn, which provides a restricted channel that further loads the diaphragm. The divergent front portion of the horn is designed for projection of the sound output in a desired beam width.
The properties of air are such that there tends to be an upper limit to the load that can be imposed upon a driver diaphragm by acoustical impedance means, and correspondingly there tends to be an upper limit to the electrical power that can be safely applied to the driver. In the present state of the art that power limit is on the order of 100 to 125 watts, with 200 watts attainable in special cases. To obtain acoustic outputs corresponding to substantially higher power inputs, it is therefore necessary to incorporate a number of electronic drivers into outdoor alerting and warning apparatus of the type here under consideration. In the usual case each of the several drivers is connected with its own individual horn throat. The several horn throats may merge into large mouth portion that is common to all of them, as disclosed in UK Pat. No. 327,145 and U.S. Pat. No. 4,344,504, but even then the apparatus has large overall dimensions. The importance of structural compactness in an outdoor alerting and warning device will be immediately apparent from the fact that such a device is ordinarily mounted on a tower to be at a level of 35 to 60 feet above the ground and must be capable of withstanding winds of the highest expectable velocities, typically taken as 145 mph. Thus, U.S. Pat. No. 4,344,504 discloses an arrangement comprising sixteen 125 watt drivers aligned vertically in a structure that was approximately 96 in. (2.44 m) high and 24 in. (61 cm) wide, producing an 800 Hz tone with a measured sound level of 127 dBA at 100 ft. However, this array was markedly directional, so that at least three more similar arrays would have been needed for an omnidirectional output, constituting an ensemble of massive dimensions that would have been subjected to correspondingly high wind loads.
To keep outdoor electronic alerting devices within size limits suitable for the wind loads they must be able to sustain, it has heretofore been conventional to incorporate no more than a limited number of drivers in such a device, sixteen being the usual maximum. What this means is that the input to such a device cannot ordinarily exceed 2 Kw, and its output is correspondingly low so that voice messages delivered by it cannot be consistently heard and understood at distances of more than about half a mile (800 m). Where greater coverage is needed, multiple units are employed, sited at uniformly spaced intervals across the area to be covered. This arrangement has several disadvantages. Its initial cost is high because of the need for multiple mounting towers. Maintenance may be complicated because of the need for periodic visits to numerous dispersed sites, as for checking the condition of the storage batteries that power each unit. Coordinating the operation of the several units presents the problem that radio transmissions may be subjected to interference and the like while wired connections are susceptible to cutting and breakage. And, in operation, a person located in hhe overlapping output patterns of two or more such units hears echo-like distortions that tend to garble spoken messages.